Cases3 cases

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1. IL-0011 HAProxy ingress piled requests onto the same pods because a Go template visited the backend map in sorted pod IP order

   Skyscanner2019

   Symptoms

   On a Sunday, a team using one of Skyscanner's Kubernetes clusters manually paged the out-of-hours Kubernetes support because their service showed elevated latency, worst at the p90 and maximum levels.

   High latencyLoad imbalanceOOMKilledIntermittent connection failures

   Trigger

   Upgrading the HAProxy ingress controller to v0.6 and going back from Service virtual IPs to individual pod IPs as backends, on the understanding that the sort-backends flag introduced in v0.5, false by default, meant backends were randomly shuffled.

   Root cause

   Software defect

   Component

   Ingress controllerHAProxyConfiguration templating

   Contributing factor

   Unrepresentative stagingUnverified assumptionIgnored early signal

   Detection

   A cluster user manually paged the out-of-hours Kubernetes support about elevated latency; Prometheus queries then showed the uneven request distribution across pods.

   Remediation

   • switched the affected service from pod IPs back to the Service virtual IP through a config change in the internal deployment tool Slingshot
   • patched the controller to keep each backend's servers in an insertion-ordered slice and changed the template to iterate over that slice
   • tested the patch in sandbox clusters and rolled it out to the production clusters

   Action items

   • Run the internal controller patch in production and return to pod IPs as the default backends
   • Raise the problem upstream with a GitHub issue and pull request
   • Test changes to cluster components against the specific behaviour and conditions that matter

   Lessons

   The team already knew that sorted backends cause hot-podding and still did not test the new flag well enough to see that it did not work; changes to cluster components need tests aimed at the exact behaviour that matters, under conditions like production. A keyword can behave differently in different parts of a language: range over a map is unordered in Go code but sorted in text/template. A small cluster component can do outsized harm to every user of the cluster and, in the end, to customers.

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2. IL-0031 Kubernetes API server OOM-killed, leaving the Skipper ingress without routes; the talk points to a misconfigured CronJob manifest

   Zalando2018

   Symptoms

   Customers were affected, and Zalando's IAM service answered requests with HTTP 404. The talk shows this on slides titled "Customer impact" and "IAM returning 404", followed by a slide on the number of pods in the cluster.

   High error rateControl plane unresponsiveOOMKilled

   Trigger

   A CronJob (apiVersion batch/v2alpha1, schedule "*/15 9-19 * * Mon-Fri") whose concurrencyPolicy: Forbid and job history limits sat inside the pod template spec instead of the CronJob spec. The talk shows it right after the slides on the number of pods and the API server being OOM-killed, and the lessons add a quota on the number of pods.

   Root cause

   Capacity exhaustion

   Component

   Kubernetes API serverIngress controller

   Contributing factor

   Single point of failureExcessive blast radius

   Remediation

   • fixed the CronJob manifest: moved concurrencyPolicy: Forbid and the job history limits to the CronJob spec, added activeDeadlineSeconds: 120 and changed the schedule to "7 8-18 * * Mon-Fri"

   Action items

   • Make the ingress stay healthy during API server problems
   • Make the ingress keep the last known set of routes
   • Use a ResourceQuota on the number of pods (the example sets pods: "1500")

   Lessons

   An AWS ALB routes traffic to all hosts when all hosts report unhealthy, so the ingress should stay healthy during API server problems. An ingress controller that reads routes from the API server should keep its last known set of routes. Use a quota for the number of pods.

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3. IL-0503 mybinder.org launches failed after a GKE node ran out of memory and its nginx-ingress proxy pod crash-looped

   Binder Project2017

   Symptoms

   On 30 November 2017 at 16:59 PST, mybinder.org operators noticed that Grafana looked wrong and that launching a new server failed with an error that a temporary user could not be created. Pods that were already running kept running, but no new ones were being created.

   Service outageContainer crash loopProbe failuresOOMKilled

   Trigger

   A build container, which ran without a memory limit, most likely used up all the memory on the node that ran the proxy pod.

   Root cause

   Capacity exhaustion

   Component

   Ingress controllerContainer memory limitJupyterHub

   Contributing factor

   No resource requests or limitsHarmful manual actionNo alert on limit

   Detection

   An operator happened to look at Grafana at 16:59 PST, saw that it looked wrong, and confirmed the error by launching a new server.

   Remediation

   • Cordoned the node that had run out of memory
   • Uncordoned the node once the proxy was working

   Action items

   • Limit the memory available to build pods
   • Change the JupyterHub configuration so user pods are not cleaned up when the jupyterhub pod restarts
   • Nominate one person to drive the response, possibly in a shared terminal, so only one person changes the cluster
   • Alert when something is wrong with the cluster, for example by counting 4xx responses on the jupyterhub pod

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